A wheel support bearing assembly for a motor vehicle includes one for a driven wheel and another for a drive wheel, and various types of bearing assemblies are provided in correspondence to respective intended uses. For example, in a conventional wheel support bearing assembly for a drive wheel shown in FIG. 6, main structure includes an inner member 50 provided with a hub axle 51 and an inner race 52, double row of rolling elements 53 and 54, an outer member 55, and a constant velocity universal joint 56 for transmitting an engine power to the hub axle 51. In the bearing assembly for the drive wheel, the hub axle 51 supporting a wheel (not shown) and a brake rotor 57 employs a medium carbon steel for mechanical structure such as S53C in the light of an easiness of forging, a cutting performance, a thermal refinement characteristic or an economical efficiency. Since achieving a reduction in size and weight of the wheel support bearing assembly of this kind including the hub axle 51 contributes greatly to an improvement of a fuel consumption and a traveling stability of the motor vehicle, it has been progressed to form a wheel mounting flange 58 of the hub axle 51 by a rib structure or to make the flange 58 thinner. On the other hand, with improvement in performance of the motor vehicle a load applied to the bearing assembly has been increased. However, a mechanical strength of the hub axle 51 itself comes close to a fatigue limit of the medium carbon steel or a raw material, and it is hard to achieve a further reduction in size and weight and a further improvement of a durability.
Particularly, in the hub axle 51, in the case of making the wheel mounting flange 58 thin for weight reduction, a rotational bending stress is concentrated to a root portion in an outboard, that is, a corner portion 61 extending to a cylindrical pilot portion 60 from a brake rotor mounting surface 59, and thus, a countermeasure is required. Accordingly, there can be considered to reduce a stress by enlarging a dimension of the corner portion 61, that is, a radius of curvature. However, this countermeasure is restricted by a possible interference with a brake rotor 57 attached to the wheel mounting flange 58. Further, the hub axle 51 for the drive wheel is provided with a through hole having a serration portion 64 in an inner diameter portion, and a constant velocity universal joint 56 has a stem having a serration in an outer peripheral surface with these serrations engaged with each other. As a consequence, the rotational bending is also concentrated on a portion between a small-diameter stepped portion on which an inner race 52 is mounted and the serration portion 64 formed in the inner peripheral surface, and hence, there is a case in that a countermeasure improving a durability is required in the serration portion 64 if an induction hardening is not applied thereto.
Under the background mentioned above, the applicant of the present invention has already proposed a wheel support bearing assembly which can achieve a weight reduction and achieve an increase of a strength of the hub axle 51 without changing a shape and a dimension of the wheel mounting flange 58. The wheel support bearing assembly is processed, as shown in FIG. 5, such that a surface hardening layer 62 is formed by an induction hardening or the like in the corner portion 61 of the wheel mounting flange 58 of the hub axle 51. Accordingly, it is possible to make the corner portion 61 of the wheel mounting flange 58 forming a weakest portion of the rotational bending fatigue high in strength thereby to improve a durability of the hub axle 51.
Further, a surface hardened layer 63 is formed by the induction hardening or the like in other portions, that is, the portions including a seal land portion a with which a seal lip of an outboard seal device comes into slidable contact, a raceway surface b, a step c and the small-diameter stepped portion d. Further, a surface hardened layer 65 is formed in the serration portion 64 formed in the inner diameter portion of the hub axle 51. By the surface hardened layers 63 and 65, it is possible to improve a rotational bending fatigue strength, an abrasion resistance, a rolling fatigue service life and the like demanded in each of these portions a to d (for example, refer to pages 4 and 5 and FIG. 2 of Japanese Laid-open Patent Publication No. 2002-87008).
As mentioned above, in the conventional wheel support bearing assembly mentioned above, it is possible to achieve the increase of the strength of the hub axle 51 while achieving the weight reduction without changing the shape and the dimension of the wheel mounting flange 58, by forming the surface hardened layer 62 in the corner portion 61 of the wheel mounting flange 58 of the hub axle 51. However, there arised a new problem that a thermal refinement deformation is generated in the wheel mounting flange 58 by the induction hardening to amplify a surface runout of the brake rotor mounting surface 59. This tendency is also caused by a thinning of the wheel mounting flange 58. This surface runout affects an oscillation of the brake rotor 57 so as to generate a brake judder, whereby a steering stability of the motor vehicle and a drive feeling are deteriorated. In this case, there can be considered a method of further turning the brake rotor mounting surface 59 after the thermal refinement of the hub axle 51 to correct deformation so as to improve the surface runout. However, since a hardness difference exists between the corner portion 61 and the unhardened brake rotor mounting surface 59, another problem arises, that is, a slight step is generated in a boundary portion between the corner portion 61 and the brake rotor mounting surface 59. Further, in the case that the surface hardened layer 65 is formed in the serration portion 64 provided in the inner peripheral portion of the hub axle 51, the serration portion 64 generates a thermal refinement deformation which in turn causes a problem that a suitable clearance (a close fit or a clearance fit) can not be secured relative to the serration of the stem of the constant velocity universal joint.
In order to solve the problem mentioned above, the applicant of the present invention has proposed a method in that an outer member or an inner member having at least the wheel mounting flange integrally formed therein is subjected to thermal refinement (for example, refer to Japanese Laid-open Patent Publication No. 2005-3061). However, there has been required a structure which more securely improves a strength and a durability of the hub axle under the rotational bending condition.